When distributing and handling heat, hot water and/or cold in a house, a vehicle, a machine or an industrial plant an accumulator tank for storing hot or cold medium is provided in most cases. In a heating system for a house the accumulator tank is normally a hot-water heater with an integrated electric heater. Several other heating systems are often connected to said system, such as a heat pump, solar heat or a pellet boiler. In vehicles such as cars, lorries or boats the cooling water of the motor and/or a burner/heater is/are often used to heat the heating medium of the accumulator tank. To heat the medium in the accumulator tank water, for example, is circulated from a heater or from the cooling system of a motor through a coil of heat-conducting material, for instance copper or stainless steel, which is arranged in a tight spiral extending through the accumulator tank so as to form a large surface for transferring heat from the water of the heat source to the medium in the tank.
When an accumulator tank is not used for some time the medium therein, usually water, will form layers with the hottest water at the top and the coolest water at the bottom. For this reason the inlet to upright or vertical accumulator tanks is often located in the lower part whereas the outlet is located in the upper part of the tank. As water is drawn from the heater new water is introduced under pressure from below and this normally creates turbulence in the tank. When water is drawn after stratification has occurred and new, cold water is supplied from below turbulence is created and the stratification is destroyed. When the heating coil is activated again it will reinforce the turbulence through its homogeneous heating of the volume where there is a heat gradient. This also causes a variation in heat-exchange efficiency along the coil. The mixing of water reduces the maximum temperature and results in inefficient heating.
The problem of cold and hot water mixing has been solved in DE102007046905 by arranging an inlet chamber below the accumulator tank and a partition wall separating the two. The heating coil is placed in the inlet chamber for heating cold inlet water. Ascending pipes of fairly good size diameter are located in the accumulator tank, which is also called a stratification chamber, and are connected to the inlet tank. Heated water from the inlet tank rises in the pipes to the stratification already established in the accumulator tank. In this way the hottest water can always be drawn from the top of the tank without the stratification being destroyed by the inlet water or the heating.
A problem with this type of tank arises when there is more than one heat source. Heat sources with a low temperature require a lower temperature of the water in the tank where the coil extends for an efficient heat exchange to occur. If the different heat sources used are, for example, a heat pump and a burner/wood boiler/pellet boiler/oil boiler where, for instance, the temperature of the outlet water from the heat pump is significantly lower than that of the boilers, it will be difficult to achieve in an efficient manner an energy exchange from the heat pump through coils in the tank if the boiler is used at the same time.
A further problem associated with prior art is that the coils used to heat the water of the accumulator tank are not very efficient, since the surface of the coils that is exposed to the water of the accumulator tank is relatively small. A solution to this problem is to use for example plate heat exchangers for the heat exchange, which increases efficiency. However, plate heat exchangers are expensive and they also cause problems as regards the stratification in the tank and may cause the water of the accumulator tank to start self-oscillating, which will destroy the stratification and reduce the heat exchange efficiency.